PROJECT ABSTRACT Pain is a fundamental protective neuronal signal for organisms to avoid danger. Nociceptors are the specific subset of peripheral sensory neurons that detect harmful/noxious stimuli and transmit pain signals to the brain. Chronic pain is a major socio-economic burden, but the underlying molecular mechanisms are not well understood. I previously found that bacterial pathogens produced pain by directly activating nociceptor neurons during infection. Moreover, I found that nociceptors played a role in suppressing local immune cell recruitment and lymphadenopathy. These findings raise the possibility that the nervous system can play a direct participatory role in host defense. Nociceptor neurons densely innervate the skin and gut, which are heavily colonized by commensal bacteria. However, the bidirectional crosstalk between the tissue-resident microbes with the sensory nervous system is poorly understood. In this NIH Director's New Innovator Award, I test the hypothesis that molecular interactions between the host microbiota and nociceptor neurons play a key role in governing pain production and the composition of the microbiota. This research is motivated by basic questions about the role of host-microbe interactions that will help us gain insights into mammalian physiology: i) Do specific commensal gut or skin bacterial species (pathobionts or symbionts) set the threshold for nociceptor neuron activity and development of chronic pain? ii) Can we identify specific bacterial molecular mediators that modulate nociceptor neural activity and pain? iii) Do nociceptor-associated ligands in spicy foods (e.g. capsaicin, mustard oil) have a significant impact on the composition and quality of the microbiota? Iv) Do nociceptor neurons produce molecular mediators that directly impact the microbiota or tissue-resident immune cells? To address these questions, I will combine neurobiological, immunological, and microbiological approaches to analyze the reciprocal interactions between nociceptor neurons and the resident microbiota. Germ-free and bacterial monocolonization experiments will determine if distinct symbiotic or pathobiotic commensal bacterial strains influence the development of pain. Neuronal calcium flux, multi-electrode array analysis, and protein chemical techniques will define the bacterial mediators that modulate nociceptor activity. These analyses will lead to the identification of potential novel molecular mediators of pain. Conversely, I will analyze if nociceptor activity in vivo plays a role in regulating the host microbiota. Using transgenic, pharmacological, and optogenetic strategies to specifically deplete, activate or silence nociceptors, I will ascertain whether sensory neurons modulate the composition of the skin and gut microbiota. Based on my foundational training in Immunology and Neurobiology, along with the ability to carry out cross-disciplinary scientific approaches, I am uniquely qualified to lead this effort. I have demonstrated a willingness to challenge convential paradigms, focusing my research on questions that have potential impacts on human health. With this proposed NIH Director's New Innovator Award, I will carry out studies to produce novel insights into host- microbe interactions facilitating the development of treatments for chronic pain and microbial dysbiosis.